Uninterruptible power supply systems, such as those used for telecommunications/data centers, often utilize batteries as the source of back-up power. Each battery typically has multiple cells or multicell modules connected in series to provide the requisite voltage, commonly referred to as a battery string. The term “cell” will be used herein to refer to both individual cells and multicell modules of a battery string unless the context dictates otherwise. The individual battery cells adjacent to each other in a section of a battery string are connected to each other by a conductive connector, such as a copper bus bar, strap, cable or the like. This connector is commonly referred to as an intercell or intercell connector. Adjacent sections of a battery string are connected to each other by a longer conductive connector, such as a cable or group of cables (that are longer than cables used for intercell connectors), referred to as an intertier or intertier connector.
Since a battery has a finite life, it will eventually fail. Consequently, battery monitors are often used to monitor the batteries in UPS systems. By detecting battery problems at an early stage before they can cause abrupt system failure, system reliability is improved.
One type of battery monitor used to monitor the batteries in UPS systems monitors the state of health of each cell in a battery string and depending on the configuration of the monitor, may monitor one or several batteries with each battery having one or more battery strings of cells connected in series. The battery strings may be connected in series, in parallel, or in a combination of series and parallel connected strings. In battery monitors available from Alber of Pompano Beach, Fla., such as the BDS series of battery monitors, the internal resistance of each cell in the battery string of each battery is measured as the internal resistance of a cell is a reliable indicator of that cell's state of health. The battery monitors also monitor other parameters, such as cell voltage, overall voltage, ambient temperature of the battery, intercell resistance, intertier resistance, discharge current, discharge events, float current, and the like. The battery monitors will alert a user if the monitored data shows a problem with the batteries being monitored. The battery monitors typically interface to a computer, local or remote, that is programmed to display the monitored data.
Battery monitors typically utilize an AC current injection method or a momentary load test method to measure battery impedance or resistance, respectively. In the momentary load test method, the battery is subjected to a momentary load (e.g., a resistance) and the instantaneous change in voltage across the battery is measured. More specifically, a momentary load is applied to the battery. This generates a test current, such as ten or twenty amps, that flows through the battery cell. The current flowing through the load (and thus also through the battery) and the voltage across the battery terminals are measured immediately prior to removal of the load. The current may be measured with an on-board current shunt in a known manner. The recovered battery voltage is then measured after removal of the load. The battery resistance is then calculated using Ohm's law by Rbatt=ΔV/I where Rbatt is the internal resistance of the battery, ΔV is the recovered battery voltage minus the battery voltage immediately before removal of the load, and I is the current flowing through the battery and load. It should be understood that the above technique can be used with an entire battery, a battery string in a battery that has a plurality of battery strings, and to individual battery cells and the use of the term battery in the description of this technique is generic to an entire battery, a battery string and a battery cell.
The battery monitors may for example utilize the teachings of U.S. Pat. No. 4,707,795 for “Battery Testing and Monitoring System” issued Nov. 17, 1987 and/or U.S. Pub. No. 2009/0224771 for “System and method for Measuring Battery Internal Resistance,” published Sep. 10, 2009, the entire disclosures of which are incorporated herein by reference.
FIG. 1 shows a prior art battery monitor 100 coupled to a battery string 102. Battery string 102 includes a plurality of battery cells 104 with adjacent battery cells connected to each other by an intercell 106. Battery string 102 may include a plurality of battery string sections 108 with adjacent battery string sections 108 connected to each other by an intertier 110. While battery string 102 is shown in FIG. 1 as having two battery string sections 108, it should be understood that battery string 102 may have more than two battery sections 108, with adjacent battery sections connected by an intertier 110, or just one battery section 108.
The positive and negative terminals of each battery cell 104 are connected to respective voltage sense leads 112 which are connected to appropriate voltage measurement inputs of battery monitor 100, which are coupled to a voltage sense circuit of battery monitor 100 which measures voltage. To simplify the figure, only three such voltage sense leads 112 are shown with only two shown coupled to battery monitor 100. Illustratively, the inputs of battery monitor 100 to which voltage sense leads 112 are connected are coupled through a multiplexer to the voltage sense circuit, allowing these inputs to be switched between positive and negative inputs of the voltage sense section. The positive terminals of each battery cell 104 are also connected to respective test load inputs of battery monitor 100 by test load leads 114. Again to simplify the figure, only two such test load leads 114 are shown. Illustratively, battery monitor 100 includes a controller 116, such as a microprocessor or microcontroller, that is programmed with software implementing the control of battery monitor 100.
Battery monitor 100 measures, among other parameters, the internal resistance of the battery cells 104 using the momentary load method as described above. Battery monitor 100 includes a load module (not shown) having one or more resistances that are selectively coupled via test load leads 114 to battery string 102, individual battery string sections 108, or individual battery cells 104 to apply the momentary load.
Battery monitor 100 also measures the intercell and intertier resistances. The flow chart of FIG. 2 shows in simplified form a method that battery monitor 100 uses to determine the resistance of intercell and intertiers. The method is described with reference to an intercell, but it should be understood that it is also applicable to intertiers.
At 200, battery monitor 100 applies a test load across a battery cell 104 and the intercell 106 connected to the negative terminal of that battery cell 104 via test load leads 114 that are connected, respectively, to the positive terminal of the battery cell 104 and the positive terminal of the adjacent battery cell 104. This causes a test current, such as ten or twenty amps, to flow through battery cell 104 and the adjacent intercell 106. At 202, battery monitor 100 measures, using voltage sense leads 112, the voltage drop across intercell 106 while the test current is flowing through the battery cell 104 and the intercell 106. Again, the test current may be measured by the monitor 100 with an on-board current shunt in a known manner. At 204, battery monitor 100 then calculates the resistance across intercell 106. More specifically, intercell resistance is computed by dividing the voltage drop across the intercell by the value of the test current in accordance with Ohm's law. At 206, battery monitor 100 checks to see if a requisite sample size of resistances has been obtained. If not, it repeats steps 200-204. If so, it then averages the samples at 208 to arrive at a final resistance of intercell 106 (Ric). The requisite sample size is the number samples so that when averaged, the resulting final resistance of intercell 106 reflects the actual resistance of intercell 106. This sample size may illustratively be determined in any known fashion, such as heuristically and may be, by way of example and not of limitation, 1024 samples.
Performing ohmic measurements on small parallel battery strings, such as those that are sometimes used in telecommunication systems, using the momentary load method can result in an error due to leakage paths of the test current used to obtain the readings, as illustrated in J. McDowall, “Parallel Strings—Parallel Universe,” (Battcon 2002). With reference to FIG. 3, a battery 300 has two parallel battery strings 304. When battery monitor 100 applies the momentary load to battery cell 302, the test current flows both in path 306, shown with solid lines, through battery cell 302 and battery monitor 100 and also in path 308, shown with dashed lines, through battery cells 302′ (which are in series with each other and in parallel with battery cell 302 when the momentary load is applied to battery cell 302) and battery monitor 100. Consequently, the resulting ohmic measurement is altered because of the parallel connection of battery cell 302 with the series/parallel connected battery cells 302′. Therefore, it is desirable to provide an improved technique for determining internal resistance of a battery cell in a battery having parallel battery strings.
This section provides background information related to the present disclosure which is not necessarily prior art.